The use of tissue transfer flaps is a method of moving tissue from a donor location to recipient location and re-attaching the arteries & veins to the blood vessels at the recipient site. These procedures enable reconstructive surgery after trauma, as well as after surgical resection of cancer. Flap transfer surgery is subject to failure via a number of modes including vascular insufficiency caused by mechanical obstruction of the artery or vein, injury caused to the transferred tissues due to the lack of blood flow during the flap transfer, or due to ischemia-reperfusion injury. The first postoperative days after free tissue transfer are characterized by the risk of microvascular complications and loss of transferred tissue by necrosis. Loss of a free flap is a devastating experience to both the surgeon and the patient. Tissue oxygenation and maintenance of microvascular blood flow in grafted tissues are crucial for flap viability. Several studies have demonstrated that frequent monitoring and early detection of compromise results in earlier intervention which reduces the number of devastating complications that lead to tissue loss. Early in the era of microsurgery, flap monitoring was performed with only clinical observation of skin color, capillary refill, and dermal bleeding. However, issues related to staffing and subjective variations in clinical assessment of a flap's perfusion have led to the search for objective methods of flap monitoring. One promising technology for measuring local tissue oxygenation in-vivo is diffuse optical spectroscopy (DOS). DOS is a quantitative near-infrared (NIR) spectroscopy technique that can determine absolute concentrations of chromophores such as oxy & deoxy hemoglobin, fat and water. Modulated Imaging (MI) is a NIR imaging method invented at BLI that is based on the principles of DOS and employs patterned illumination to interrogate biological tissues. This non-contact approach enables rapid quantitative determination of the optical properties and in-vivo concentrations of chromophores over a wide field-of-view. The central aim of the proposed research is to further the development of Modulated Imaging and to assess the viability of this as a means to determine status of tissue reconstruction flaps. In Phase I, we carried out an in-vivo MI study using a dorsal pedicle flap rodent model. The dorsal pedicle flap is easily implemented to establish controlled ischemia and re-perfusion of the wounds. This allowed us to employ MI to deduce spatially resolved maps of tissue hemoglobin, oxygenation and hydration over the course of several days. In Phase II we propose to develop and validate an MI instrument for clinical use. Investigations will first evaluate the performance of MI in a controlled model of partial vascular congestion using adult Yorkshire pigs. This will be followed by a study in which MI and a potentially competing FDA cleared device will be employed in a clinical situation in order to assess local flap status. In parallel with the Phase II research outlined herein, we will aggressively pursue commercialization of a medical device based on MI.